1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same, in particular to a zoom lens suitable for use in a compact digital camera.
2. Description of the Related Art
In recent years, digital cameras that pick up an image of an object using a solid state image pickup element such as a CCD or CMOS have replaced film cameras and become the mainstream. Furthermore, various categories of digital cameras ranging from popular-priced compact type cameras to function-rich cameras for professionals have been developed. In the present invention, compact, popular-priced cameras are particularly focused on.
Users of such popular-priced compact digital cameras generally wish to enjoy easy and simple shooting in various shooting situations anywhere at any time. For this reason, such users favor small size digital cameras, especially cameras that are small with respect to the thickness direction and can be conveniently carried in a pocket of clothes or a bag. Therefore, a further reduction in the size of the taking lens system is demanded.
As to the angle of view, a wider angle of view is desired. Therefore, low-price, bright (or fast) zoom lenses with high optical performance having a high zoom ratio exceeding 3 and a zoom range reaching to a wide diagonal angle of view larger than 70 degrees are demanded.
As a prior art zoom lens that is bright and has a relatively wide angle of view, a zoom lens including in order from the object side a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a positive refractive power, and a fourth lens unit has been known as disclosed in Japanese Patent Application Laid-Open No. 2009-163222.
The optical system disclosed in Japanese Patent Application Laid-Open No. 2009-163222 has a compact overall size while having a wide angle of view and a high zoom ratio of approximately 4. To achieve these specifications, lenses having high refractive powers are used in the zoom lens. In particular, the negative first lens in the first lens unit having a negative refractive power has a high refractive power. To achieve this, a grass material having a high refractive index is used in the optical system.